Dye diffusion thermal transfer printing is a well known process in which one or more thermally transferable dyes are transferred from selected areas of a dyesheet to a receiver material by localised application of heat, thereby to form an image. Full colour images can be produced in this way using dyes of the three primary colours, yellow, magenta and cyan. Printing is conveniently carried out using a dyesheet in the form of an elongate strip or ribbon of a heat-resistant substrate, typically polyethylene terephthalate polyester film, carrying a plurality of similar sets of different coloured dye coats, each set comprising a panel of each dye colour (e.g. yellow, magenta and cyan plus optional black), with the panels being in the form of discrete stripes extending transverse to the length of the ribbon, and arranged in a repeated sequence along the length of the ribbon.
Dye diffusion thermal transfer printing may be used to print directly onto a variety of substrates, for example onto PVC. However some substrates, e.g. polycarbonate, certain polyesters and ABS, are not sufficiently dye receptive for good quality images to be formed by printing onto them directly.
This problem is well-known and one known solution is to apply a dye-receptive coating, also called a receiver layer, during manufacture of the substrate.
In order for such coatings to adhere to the substrate they must be sufficiently adhesive. However, as dye diffusion thermal transfer printing involves the physical contact of the printing ribbon with the substrate to be printed on, this can create difficulties with excessive ribbon release force or even ribbon sticking.
To overcome this problem such coatings are typically curable so that their adhesive nature is reduced during cross-linking without the risk of the coating detaching from the substrate. To further reduce the risk of ribbon adhesion it is known to incorporate in the coating so-called release agents, e.g. silicone oil. However, often only a small region of the substrate is to be printed on and so coating the substrate during its manufacture can involve unnecessary costs.
An alternative solution is to transfer a receiver layer to the substrate by the application of heat. Often this involves the thermal transfer of a dyable resin with excellent adhesive properties in order that it adheres to the substrate. In this case, the receiver layer is typically not cured as curing during the coating process, i.e. prior to transfer, would hinder or prevent the transfer of the receiver layer onto the substrate. To reduce ribbon release force upon subsequent printing, release agents may be used but this often provides an insufficient reduction in the ribbon release force and problems of ribbon sticking are not eliminated, particularly where a receiver layer having excellent adhesion is used.
As a solution to this problem it has been suggested to thermally transfer two or even three layers. For example an arrangement involving an adhesive layer followed by an image-receiving layer and an uppermost release layer has been proposed in EP 0474355.